JP-A-6-256518 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")(corresponding to U.S. Pat. No. 5,449,734) discloses a curable composition which undergoes condensation reaction and addition reaction in one pot to provide a cured product having a silicon type IPN composed of a network silicon skeleton and a silicon-containing polymer, e.g., polycarbosilane or polysiloxane.
However, the publication shows no guideline for securing the compatibility between a system curing on condensation and a system curing on addition, which seems to be an important prerequisite for IPN formation, and homogeneity of a cured product obtained after curing. Neither does it teach a process for producing a thick-walled molded article having practical mechanical characteristics sufficient for use as heat-resistant structural material.